1. Field of the Invention
The invention generally relates to the field of wastewater treatment. A method of treating wastewater using an anaerobic bioreactor which contains a “bio-nest” structure to retain the microbial content and improve digestion efficiency is disclosed. The system is particularly useful for treatment of lipid rich wastewater.
2. Description of the Related Art
Improvement of the wastewater treatment process, and in particular, dairy and agricultural wastewater treatment, is an important goal. This is particularly important since the U.S. Environmental Protection Agency (EPA) is putting new rules into place regarding agricultural waste. All large Concentrated Livestock Animal Feeding Operations (CAFOs) will soon be required to obtain permits that will ensure they protect America's waters by keeping wastewater and manure out of the nation's waterways. The rule will control runoff from agricultural feeding operations, preventing billions of pounds of pollutants from entering America's waters every year. Likewise, similar environmental quality issues are becoming increasingly important in other countries outside the United States as well. Accordingly, new methods for clearing wastewater of biological pollutants are needed in order for agricultural facilities to be compliant with the new ruling.
Conventional wastewater treatment facilities typically utilize an upflow anaerobic sludge blanket (UASB)-type system. The UASB high rate reactors are based on the difference of solid and liquid retention time by applying a “granule blanket” (Lettinga et al., 1980; Marin et al., 1999; each of which is incorporated by reference herein in its entirety). However, the UASB granule blanket reactors have some limitations because the granulation formulation depends on wastewater types rather than reactor design and operation (Jhung J. K. and Choi E. 1995, Batstone and Keller, 2001; each of which is incorporated by reference herein in its entirety). For example, granule formation can be difficult to form during UASB treatment of wastewater containing a high concentration of fat and lipids such as dairy wastewater (Hawkes et al., 1995). Accordingly, UASB methods of treating lipid-rich wastewater have often met with poor results (Hawkes et al., 1995, Perle et al. 1995, Petruy and Lettinga 1997, Vidal et al., 2000; each of which is incorporated by reference herein in its entirety).
The poor performance of UASB systems is often due to the binding of the wastewater lipids to the granules, causing breakdown of the granules. The broken down granules can then float and wash out of the system (Perle et al. 1995; Petruy and Leffinga, 1997). Further, Alphenaar and Lettinga (1993; the disclosure of which is incorporated by reference herein in its entirety) concluded that UASB reactors were not suitable to be applied if lipids contribute 50% or more of the chemical oxygen demand (COD) of wastewater. Lipid loading rates exceeding 2–3 kg COD m−3 day−1 caused granule breakdown resulting in total sludge wash-out. Even at lower loading rates (less than 1.5 g/l/d), the system was still unreliable because of unpredictable sludge flotation.
Conventional UASB systems often also have a “dead zone” or shortcut of wastewater flow (Lens et al, 1998). Although a relatively high organic loading rate (OLR) of more than 20 kg COD/m3/day could be achieved in UASB systems with high strength wastewater (COD>2500 mg/l ) on a laboratory scale, full scale UASB systems were generally designed not to exceed 15 kg/m3/day at an optimum temperature 35° C. (Driessen and Yspeert, 1999; incorporated by reference herein in its entirety). Additional concerns with the UASB reactors include mass transfer limitations due to sub-optimal mixing condition inside the reactor, and high upflow velocity and gas production causing granules wash-out (Driessen and Yspeert, 1999). An increase in upflow velocity in such systems may cause too little contact time between the granule and the soluble substrate. Also, a substantial “dead zone” was present in the reactor, containing a low amount of sludge and/or poor mixing conditions (Lens et al, 1998, Chernicharo and Cardoso, 1999; each of which is incorporated by reference herein in its entirety).
UASB systems are often plagued with poor reliability, particularly when used for high lipid wastewater. In continuous UASB systems, the scum layer and sludge layer on the top of the reactor were subsequently washed out (Rinzema, 1988; Yang, 1994; each of which is incorporated by reference herein in its entirety). The solids content accumulated in the sludge bed, which led the poor reliability of operational performance. In order to solve these problems, several researchers (Sayed, 1987; Lettinga and Hulshoff Pol 1991; each of which is incorporated by reference herein in its entirety) suggested using the flocculants sludge instead of granules. Intermittent (rather than continuous) feeding operations were suggested (Nadais et al., 2001; the disclosure of which is incorporated by reference herein in its entirety).
Several UASB modifications, such as upflow anaerobic buffer reactors (UABR) and upflow anaerobic filter reactor results were reported for the application for treating milk wastewater (Hawkes et. al., 1995; Cordoba et. al., 1995; Vartak et. al., 1998; each of which is incorporated by reference herein in its entirety). Rather than granule blanket sludge, floc sludge was applied in these modified reactors. The main advantages of these modified reactors included the maintenance of a high sludge concentration and the prevention of sludge wash out by the use of filters, media and baffles.
The presence of a high lipid content in some types of wastewater has been found to cause the inhibition of methanogenic activity (Perle et al., 1995). This inhibition of methanogenic activity resulted in poor anaerobic biodegradation of fat and lipids in the wastewater.
An additional issue with conventional wastewater treatment systems is that some systems require a heating method to control the temperature for optimal performance, particularly for wastewater having a high lipid content. However, although heating the system may improve lipid biodegradation, the economic cost can be very high, particularly if an external heat source is not available (Petruy and Lettinga, 1997).
What is needed in the field of wastewater treatment is an alternative, reliable method to efficiently treat wastewater, particularly wastewater that is high in lipid content such as dairy wastewater.